Use of agglomerates of silicic acid as fillers in dental materials

ABSTRACT

The invention relates to the use of agglomerates of silicic acid having an average agglomerate size of 0.5 to 50 μm, containing primary particles of silicic acid having an average particle size of 1 to 100 nm and serving as filler in dental materials on the basis of ethylenically unsaturated polymerizable monomers. This reinforcing filler is practically colorless and can be blended in considerable quantity into the polymerizable monomer material especially mono-functional and di-functional as well as polyfunctional esters of the acrylic acid and the methacrylic acid and yields a satisfactorily processable plastically deformable paste. If necessary, the agglomerated silicic acid is used together with non-agglomerated micro-fine silicic acid of an average particle size of 1 to 100 nm. The shaped bodies obtained after the curing of the polymerizable material with the fillers show superior compressive strength, low thermal expansion, high colour stability and low water absorption, and moreover can be finished to high luster. The agglomerated fillers provided according to the invention are generally suitable for tooth filling preparations and tooth restoration materials as well as for the production of artificial teeth.

As it is known from the U.S. Pat. No. 3,066,112, to the hardenabledental materials on the basis of ethylenically unsaturated,polymerizable monomers of which especially the mono-functional anddi-functional as well as poly-functional esters of the acrylic acid andmethacrylic acid have achieved considerable practical use, higheramounts of inoganic fillers are added for the reduction of the shrinkageduring polymerization, decrease of the thermal expansion coefficient andincrease of hardness of the obtained polymerisates. The fraction ofinert inorganic fillers can amount up to more than 80% of the totalmaterial in such dental permanent filling materials. As fillers e.g.quartz, quartz glass or silicate glasses like lithiumaluminium silicateor barium silicate glass are used as fine powder. The particle sizes ofthese fillers are within the range of 1 μm to approximately 100 μm, theaverage particle diameter being generally in the magnitude ofapproximately 10 μm.

Disadvantageous upon use of these fillers is the fact that the dentalmaterials produced therefrom do not yet have a satisfying abrasionresistance and have surface roughness upon utilisation as toothfillings. Due to the mediocre abrasion resistance, the tooth fillingmaterials could not be used with the mentioned fillers in the molarfield where the amalgam fillings are still predominant despite thetoxicologically critical mercury.

The surface roughness of the mentioned materials leads additionally toproblems upon use in the anterior tooth area since the depositing ofdental plaque is promoted, thereby causing discolorations as well asmarginal secondary caries.

From the DE-OS No. 24 03 211, there is known that very finely dividedfillers, like micro-fine silicic acid (silicon dioxide gel) or aluminiumoxide, having particle sizes within the range of 5-700 nm, yieldwell-polishable shaped bodies after curing of the dental materialproduced therewith. As can be seen from the articles of R. Schafer in"Dental Echo", Vol. 49, 136 (1979) and A. Groβ in "Chemie in unsererZeit", 13th Annual Set, page 142 (1979), no workable pastes having highcontents of fillers as required for the dental application in the dentalpractice can be obtained with these micro-fine fillers which have a verylarge specific surface. It has been shown that upon blending of themicro-fine silicic acid, e.g. of the pyrolitically produced silicondioxide gel, into the liquid monomer, a thickening of this monomer to ahoney-like, stringy and adhesive mass is firstly obtained and then issuddenly converted into a bittle mass upon continuous addition ofsilicic acid which mass finally dissolves in a freely flowing powder.Thus, only masses of salve-like or powdery consistency can be produced.In these references, it is now pointed out that the micro-fine fillershall be firstly premixed and prepolymerised with a polymerizablemonomer. The solid product so obtained is crushed and finely ground.This pulverized splinter polymerisate is then used as actual filler inthe hardenable dental material.

In this manner, however, very high fractions of inorganic fillers arenot achievable in the dental materials since, according to this knownmethod the splinter polymerisate to be incorporated in the monomerconsists considerably of organic substance, already.

Therefore, upon utilisation of this filler, a higher thermal expansioncoefficient must be accepted thereby partly compensating the aspiredadvantage of having a high filler fraction within the polymerizabledental material. In case the filling material and the tooth substancehave a considerably different thermal expansion coefficient, theso-called "pump effect" is obtained caused by the change of temperaturee.g. by ingestion. Through the formed marginal gap, bacteria canpenetrate and lead to secondary caries or to damages of the pulp in deepcavities. In addition, the micro-fine silicic acid must be mixed veryintensively with the monomer fluid during the production of thepre-polyfmerisate in order to achieve a uniform coating of the particlesof the silicon dioxide powder with the hardenable monomer. Thecold-hardening monomer preparations are hardly suitable due to therelatively rapid course of polymerization. Therefore, the method of thehot-polymerization must be used. The relatively high temperaturesobtained therewith lead, however, to discolorations since the curingshall occur rather intensively in order to obtain a hard and brittlefilling material which is easily pulverable. Due to this more or lessconsiderably yellowish colored pre-polymerisate powder, it is difficultto obtain dental materials in light coloring and to secure a uniformcolour adjustment during the technical production. Although themicro-fine, pyrogenic silicic acid has been treated with silanisingagents to improve the adhesion of the inorganic fillers with the polymerthereby obtaining a certain agglomeration of the powder particles, thesefillers, however, have merely a rather poor hardness, since theseagglomerates are not hard and resistant because the cohesion by thesilanising agents is not stable. Upon incorporation of theseagglomerates into the monomer preparation, a renewed division occurs,and again only salve-like or powdery products are obtained.

From the DE-OS No. 14 67 437, relatively stable agglomerated silicicacids are known; their advantageous utilisation for the specialrequirements of dental materials is, however, in no way disclosed sinceonly the application as delustrant for lacquer is described.

Therefore, it is the object of the invention to find a new reinforcingfiller for dental materials essentially consisting of inorganic materialwhich filler is practically colourless and can be blended inconsiderable quantity into the polymerizable monomer materials. In thisconnection, pastes shall be obtained which are plastically deformable inan easy way which pastes yield to well polishable shaped bodies of lowthermal expansion coefficient and high abrasion resistance after thecuring practically without shrinking.

According to the invention, this is achieved by using an inorganicfiller in form of agglomerate of silicic acid having an average particlesize of 0.5-50 μm, preferable 1-10 μm, which filler consists of primaryparticles of silicic acid having an average particle size of 1-100 nm.If necessary, the granular material can contain additional inorganic,oxidic constituents as e.g. aluminium oxide or boron oxide.

On the one hand the agglomerate material must be so stable that aredivision does not occur during the processing with the monomers and onthe other hand should be hard to such a degree only that the shapedbodies are still polishable after polymerizing. The hardness of thefiller can be adjusted in the course of the subsequently describedmethod of production by the duration and especially by the temperatureof the final annealing; therewith the suitable range of temperatureamounts from approximately 600° C. up to more than 1200° C. Naturally,the temperature and annealing period must be adjusted to the respectivecombination silica gel/oxidic binding agent in order to achieve thedesired values of hardness. This can be determined by the expert throughsimple tests.

The agglomerates of silicic acid can be produced with or without bindingagent in different ways. For a cohesion of the primary particles withsilicon dioxide, a method is suitable in which the micro-fine silicicacid which is obtained through precipitation or preferably throughpyrogenic steps and whose particle size can amount between 1 to 100 nm,preferably between 5 to 50 nm, is premixed with a water glass solution,the mixture being then acidified, the mass being dried and slowly heatedup to more than 600° C. Afterwards, it is washed with water therebyremoving soluble fractions. After drying, the agglomerated material soobtained is adjusted to the requested granular size by milling andscreening, if necessary.

In similar manner, a cohesion and stable agglomerating to an inorganicagglomerated material of silicic acid can be achieved which is suitableas filler by using a boric acid solution or alcoholic aluminiumalcoholate solution. While the aluminium compounds cause theagglomeration probably through the formation of the binding agentaluminium oxide, the boron compounds obviously have primarily acatalytic effect for the cohesion of the primary particles within theagglomerated material, wherein the major part of the formed boron oxideis volatilized during the step of annealing without influencing thestability of the agglomerate. Although it was known from the DE-OS No.27 16 225, to impregnate precipitated silicic acid with boric acid; thesilicic acid serving as catalyst support for the boron oxide and asfiller for organopolysiloxanes which are used as so-called bouncingputties.

This state of art, however, could thus not give any suggestion for theadvantageous use for the special requirements of dental materials ofthis agglomerated material of silicic acid bound by boric acid.

According to another suitable method of production, the raw material ispremixed with silicon tetrachloride and then is hydrolysed in moistatmosphere or by addition of water. The mixture obtained is slowlyheated up to more than 600° C. wherein the hydrochloric acid formedevaporates. Afterwards, residual HCl is washed out and the requestedagglomerate size is adjusted through milling and screening.

This method can be combined in such a manner that silicon tetrachlorideis added to the mixture of the primery silicic acid and water glass. Thefurther processing is then performed as described by drying and heatingand subsequent washing out of soluble constituents as e.g.alkalichloride.

According to a further advantageous manner of production, the micro-finesilicic acid is premixed with organo-silicon compounds which preferablycontain a plymerizable residue. If necessary, a polymerization catalystin conventional concentration is added and the mixture is polymerizede.g. under action of heat. Afterwards, the mixture is slowly heated onair until the organic constituents are burned, and finally brought up tomore than 600° C. After cooling down, the requested agglomerate size canbe obtained through milling and screening.

Suitable organosilicon compounds are e.g.: vinyl trichlorosilane, vinyltrimethoxysilane, allyldimethylchlorosilane,γ-methacryloxypropyltrimethoxysilane,β-(3,4-epoxycyclohexyl)-ethyl-trimethoxysilane andγ-glycideoxypropyltrimethoxysilane. Even longer annealing of themicro-fine silicic acids above 800° C. leads to useful agglomeratedmaterials without any binding additives.

The agglomerated material of silicic acid according to the invention canbe easily mixed with the liquid polymerizable monomers. In thisconnection, the mono-functional, di-functional or polyfunctionalderivates of the acrylic acid of methacrylic acid have proven useful asmonomers, especially the esters. In advantageous manner, the filleraccording to the invention is used together with a fraction ofnon-agglomerated, micro-fine silicic acid of a particle size of 1-100nm, preferably of 5-50 nm for adjustment of the consistency and forprevention of separation. Relative to the total material, the fractionof the material of the non-agglomerated silicic acid can be between 0.5and 40%, preferably between 2-30% by weight.

The inorganic fraction of filler in the dental materials according tothe invention shall amount as usually between 10 and 80%, preferablybetween 30 and 80% and especially preferably between 50 and 70% relativeto the total material.

Prior to its application the agglomerated material of silica gel can besilanized to improve the cohesion with the polymer e.g. by treatmentwith trimethoxy-(3-methacryloxypropyl)-silane. For dental use, knownorganic or inorganic pigments and/or opacifiers are usually added to thematerial for matching to the natural teeth. As hardening catalysts i.a.organic peroxides like dibenzoyl peroxide, or azo-compounds likeazo-bis-iso-butyronitrile can be used in these preparations. The redoxsystems, as e.g. dibenzoyl peroxide/N,N-bis-2-hydroxyethylxylidine ordibenzoyl-peroxide/barbituric acid derivates which redox systems aresuitable for the cold hardening of vinyl-unsaturated monomers, are alsoappropriate.

As hardening catalysts, also substances can be employed, which initiatethe polymerization after irradiation with UV-light or visible light ase.g. benzoinalkylethers, benzilmonoketales or aliphatic and aromatic1,2-diketone compounds wherein the photopolymerization can beaccelerated in a manner known per se through addition of activators likeamines or organic phosphites.

After curing the polymerizable materials with the fillers according tothe invention, the compressive strength of the shaped bodies is up tomore than 20% higher than upon usage of fillers according to the stateof art. The compressive strength amounts in the photopolymerisates up tomore than 450 MPa instead of approximately 350 MPa. Moreover, thepositive properties of the quartz filled shaped bodies, namely lowthermal expansion, high colour stability, low water absorption arecombined with the positive properties of the pre-known shaped bodiesproduced with micro-fine silicic acid, with regard to the polishingability, in one material without obtaining any disadvantages. It couldnot be expected that through the usage of mechanically relatively stableagglomerates from the micro-fine silica gels and possibly purelyinorganic binding agent as filler, in combination with polymerizablemonomers, paste-like dental materials are obtained which have goodconsistency despite a high fraction of filler and are finishable to highluster after curing.

The agglomerated filler according to the invention is generally suitablefor tooth restoration compositions, and thus not only for tooth fillingpreparations but also for such purposes for which such materials areusually used advantageously, e.g. for production of crowns, bridges,veneers and alike prosthetic dental appliances and also for theproduction of artificial teeth.

EXAMPLE 1 Agglomeration with water glass

The quantity of silica gel as mentioned below is kneaded into 100 gfiltered soda water glass (approximately 39° Baume) and the mixture isadjusted to pH 5-6 with concentrated hydrochloric acid. Through additionof methanol, a kneadable consistency is obtained. After kneading for twohours, the methanol is removed at 60° C. and 200 torr and dried throughheating at 120° C. Then, it is brought slowly to 800° C., the cooledproduct is ground and freed from soluble content of alkali by washingwith water. Through repeated drying at 120° C., a pulverant filler isobtained (average agglomerate size approximately 5 μm).

    ______________________________________                                                  primary silica gel                                                                                 average                                                              specific particle                                                   mode of   surface  diameter                                                                             quantity                                Number of product                                                                         production                                                                              (m.sup.2 /g)                                                                           (nm)   (g)                                     ______________________________________                                        1           pyrogenic  50      40     60                                      2           pyrogenic 380       7     25                                      3           precipitated                                                                            110      28     60                                      ______________________________________                                    

EXAMPLE 2 Agglomeration with boric acid

A saturated, aqueous solution of boric acid is produced by decantingfrom the bottom sediments at approximately 78° C. In 468 g of thissolution, 1400 g pyrogenic silicic acid (specific surface 50 m² /g,average particle diameter 40 nm) are introduced under stirring at atemperature of above 80° C. during approximately 2 hours. Subsequently,it is stirred for 30 minutes and then the water is removed in thevacuum. The solid substance is dried at 120° C. and slowly heated up to800° C. After milling in a ball mill, the obtained powder is extractedwith water until the wash water is free of boron. Subsequently thefiller is dried at 120° C. in the vacuum.

The average agglomerate size of the filler is approximately 4 μm.

EXAMPLE 3 Paste/Paste-preparation for production of tooth fillings

For production of the pastes, there are kneaded:

(A) 249.0 g of agglomerated silicic acid coloured tooth-like (averageparticle size 5 μm, average primary particle size 40 nm, binding agentSiO₂, <63 μm, silanized) and

33.0 g pyrogenic, non-agglomerated, silanized silica gel with a solutionof

1.5 g N,N-bis-hydroxy-ethyl-3,5-di-t-butyl-aniline in 122.5 g2,2-bis[p-(γ-hydroxy-propoxy-)phenyl]-propane dimethacrylate and

31.0 g bis-GMA

(B) 287.0 g of the silanized agglomerated silicic acid used in paste A)and

42.0 g pyrogenic, non-agglomerated, silanised silica gel with a solutionof

4.0 g p-chlorobenzoylperoxide in 200.0 g2,2-bis[p-(γ-hydroxy-propoxy-)phenyl]-propane dimethacrylate and

40.0 g bis-GMA

By mixing equal parts of both pastes, a plastic mass is obtained whichcan be introduced and shaped easily in conventionally prepared toothcavities. Approximately 2 minutes after mixing, the hardening startswhich ends after approximately 3.5 minutes. The abrasion resistantfilling can be finished to high luster by conventional dentalprocedures. The linear thermal expansion coefficient of the curedmaterial is 45×10⁻⁶ K⁻¹.

EXAMPLE 4 Tooth filling material polymerizable by UV-light

A solution is prepared of:

20.0 g bis-hydroxymethyl-tricyclo[5.2.1.0.²,6 ]-decane-di-methacrylate(stabilised with 200 ppm p-methoxyphenol and 200 ppm jonol)

120.0 mg benzildimethylketale and

100.0 mg didecyl-phenyl-phosphite

Furthermore, a powder mixture is prepared of

14.0 g silanized agglomerated silicic acid according to the invention(average particle size 6 μm, average primary particle size 40 nm,binding agent B₂ O₃, <63 μm) coloured tooth-like

10.0 g silanized, non-agglomerated, pyrogenic silica gel and

2.0 g finely pulverized calciumfluoride.

10.5 g of the solution and 18 g of the powder mixture are kneaded to ahomogenous tooth filling material. After an exposure time of 20 sec.with a conventional UV-irradiation equipment (UVIOLITE, Firma ESPE) withan output of 70 mW the material is cured in a layer thickness ofapproximately 3 mm. The abrasion-resistant material is finishable tohigh luster and has a linear thermal expansion coefficient of 45×10⁻⁶K⁻¹ :

The compressive strength is 400 MPa measured at exposed test samples of2×2×4 mm after storage under water at 36° C. for 24 hours.

COMPARATIVE EXPERIMENT 1

In a mixture of

14.0 g bis-hydroxymethyl-tricyclo[5.2.1.0.²,6 ]-decane-diacrylate(stabilised with 200 ppm-p-methoxyphenol and 200 ppm jonol)

6.0 g bis-GMA

9.0 g silanized, pyrogenic silicic acid (specific surface 50 m² /g)

0.3 g methyl-diethanolamine-dimethacrylate and

0.03 g camphor quinone

a quantity of commercial silicic acids on the one hand and ofagglomerated material according to the invention on the basis of thesesilicic acids on the other hand is introduced which quantity isdetermined by preliminary tests to result in a paste like material. Thetest samples obtained after exposure with a commercial dentalirradiation equipment emitting visible light (ELIPAR-equipment, FirmaESPE) show the physical properties listed in the following table.

    __________________________________________________________________________    SILICIC ACID                                Inorganic                               average             Surface                                                                             Thermal                                                                              Flexural                                                                           fraction                                particle     Workable as                                                                          hardness                                                                            expansion                                                                            strength                                                                           [% of total                       type  size [nm]                                                                           agglomerated                                                                         tooth filling                                                                        [MPa] [K.sup.-1 ]                                                                          [MPa]                                                                              material]                                                                           REMARKS                     __________________________________________________________________________    pyrogenic                                                                           40    no     no     128   62 × 10.sup.-6                                                                 65   58    Commercial                                     (stringy)                      product "AEROSIL                                                              OX50" Firma DEGUSSA         pyrogenic                                                                           40    yes    good   202   52 × 10.sup.-6                                                                 85   60                                pyrogenic                                                                           7     no     no     160   78 × 10.sup.-6                                                                 72   32    Commercial                                     (stringy)                      product "AEROSIL                                                              380" Firma DEGUSSA          pyrogenic                                                                           7     yes    good   255   46 × 10.sup.-6                                                                 93   68                                precipi-                                                                            8     no     no     132   102 × 10.sup.-6                                                                81   22    Commerical                  tated              (stringy)                      product "FK310"                                                               Firma DEGUSSA               precipi-                                                                            8     yes    good   191   48 × 10.sup.-6                                                                 114  70                                tated                                                                         __________________________________________________________________________

Only the use of the agglomerated silicic acids according to theinvention yields pastes which can be processed as tooth fillings. Thesurface hardness of the polymerisates is increased to more than 50%, thethermal expansion is decreased about 15-50%, the flexural strength isincreased to more than 30%.

COMPARATIVE EXPERIMENT 2

The physical data of filling A (according to the invention, example 3)are compared with the commercial preparations B, C and D, whose basisare the following fillers:

B--glass powder, average particle size approximately 10 μm (AdapticRadiopaque, Johnson & Johnson)

C--glass powder, average particle size approximately 10 μm mixed withmicro-fine silicic acid (Miradapt, Johnson & Johnson)

D--polymer covered micro-fine silicic acid, average particle sizeapproximately 40 nm (Silar, 3M Co.)

    ______________________________________                                        Preparation                                                                               A      B       C       D                                          ______________________________________                                        Compressive strength                                                                        370      240     320   290                                      [MPa]                                                                         Flexural strength                                                                            80       65      85    50                                      [MPa]                                                                         Stability of colour                                                                         superior good    good  poor                                     Polishability superior bad     poor.sup.⊕                                                                      superior                                 ______________________________________                                         .sup.⊕  = only with special grinding instruments                     

The tooth filling A according to the invention combines the advantageousproperties of conventional materials with the glass fillers (preparationB) as well as with the polymer covered micro-fine silicic acids(preparation D); this result is not achieved by the mere mixture of bothfiller types (preparation C).

Only the material according to the invention is polishable together withhigh compressive strength and flexural strength and has superior colourstability.

We claim:
 1. A dental material comprising a polymerizable monomer and aninorganic filler, said filler comprising agglomerates of silicic acidhaving an average agglomerate size of 0.5 to 50 μm, said agglomeratescomprising primary particles of silicic acid having an average particlesize of 1 to 100 nm.
 2. A dental material according to claim 1, whereinthe average agglomerate size of said agglomerates is 1 to 10 μm.
 3. Adental material according to claim 1, further comprisingnon-agglomerated, micro-fine silicic acid having an average particlesize of 1 to 100 nm.
 4. A dental material according to claim 3, whereinsaid non-agglomerated silicic acid has an average particle size of 5 to50 nm.
 5. A dental material according to claim 3, wherein saidnon-agglomerated silicic acid comprises 0.5 to 40% by weight of thetotal material.
 6. A dental material according to claim 5, wherein saidnon-agglomerated silicic acid comprises 2 to 30% by weight of the totalmaterial.
 7. A dental material according to claim 1, wherein saidinorganic filler comprises 10 to 80% by weight of the total material. 8.A dental material according to claim 7, wherein said inorganic fillercomprises 30 to 80% by weight of the total material.
 9. A dentalmaterial according to claim 8, wherein said inorganic filler comprises50 to 70% by weight of the total material.
 10. A dental materialaccording to claim 1, wherein said polymerizable monomer comprises amono-functional, di-functional or poly-functional derivative of acrylicor methacrylic acid.
 11. A dental material according to claim 10,wherein said derivative is an ester.
 12. A dental material according toclaim 1, wherein the average agglomerate size of said agglomerates isabout 5 μm and the average particle size of said primary particles isabout 40 nm.
 13. A dental material according to claim 1, wherein theaverage agglomerate size of said agglomerates is about 6 μm and theaverage particle size of said primary particles is about 40 nm.
 14. Acured, shaped body prepared from a dental material as claimed inclaim
 1. 15. A dental filling material according to claim 1, whereinsaid agglomerates of silicic acid are silanized.
 16. A dental fillingmaterial according to claim 3, wherein said non-agglomerated, micro-finesilicic acid is silanized.